Women have a greater prevalence rate of knee osteoarthritis than men do. Although some risk factors for knee osteoarthritis in women have been qualitatively identified, limited studies exist regarding the quantitative nature of risk factors for knee osteoarthritis. Particularly limited are the studies addressing the differences in lower extremity motion patterns between women and men in daily functional activities, and the effects of the motion patterns on the risk of knee osteoarthritis in women. Our recent studies suggest that women may have different knee motion patterns from men, and that stair climbing may be a more sensitive evaluation procedure for knee disorders. The objective of our long-term research project is to identify modifiable motion related risk factors for knee osteoarthritis in women and develop corresponding prevention strategies. The purpose of the proposed study is to obtain basic understanding of the differences in knee motion patterns between women and men in selected daily functional activities such as level walking and stair climbing, and the effects of estrogen level on women's knee motion patterns in these activities. The knee flexion angle, valgus-varus angle, internal rotation angle, flexion-extension moment, and valgus-varus moment will be used as knee motion measures. Knee static alignment angle will also be measured. The specific aims of this study are: Specific Aim l: To compare the knee motion patterns between women and men during the stance phases of level walking and stair climbing including ascending and descending. Specific Aim 2: To determine the effect of women's estrogen level on the women's knee motion patterns in level walking and stair climbing. Specific Aim 3: To compare static knee alignment angle with weight bearing between women and men. Specific Aim 4: To determine the relationship between static knee alignment angle with weight bearing and maximum knee valgus or varus angle in level walking and stair climbing.